In a receiver of digital radio communications, the demodulation of the signals is carried out in digital. Before carrying out this demodulation, an analog stage of the receiver brings the signal around an intermediate frequency. The objective of this analog stage is to amplify the signal and to filter it with the aid of an antialiasing filter. After filtering, the signal is digitized using an analog-digital converter usually given the acronym CAN.
In the nominal receiving conditions with no interference, the analog-digital conversion of an analog signal needs only a reduced number N of bits. This makes it possible to limit the complexity of the digital processings carried out on the digitized signal. If the power of the signal at the input of a CAN converter is set to the maximum possible conversion efficiency R, the loss introduced by the digitization is limited. For N=3, the loss due to the digitization does not exceed 0.2 dB. The conversion efficiency R is the ratio between the signal-to-noise ratio before conversion and the signal-to-noise ratio after conversion.
In order to control the gain of the analog chain so as to have a maximum efficiency of the CAN converter, an adaptive gain control loop AGC, also called the automatic gain control loop, is usually used. FIG. 1 gives an example of the receiving chain comprising a AGC loop. This receiving chain comprises an analog stage 113 and a digital stage 114. The analog stage comprises notably a variable gain amplifier 100 having as its objective the amplification of an analog input signal e. After amplification, the signal is inserted into a CAN converter 101 on N bits. The digitized signal corresponds to a succession of digital samples encoded on N bits. It is filtered with the aid of an interference-suppressing digital filter 107, for example of the finite impulse response FIR filter type. The bits at the output of the filter are rescaled 108 that is to say encoded on a smaller number of bits, the rescaling operation usually being called “bit rescaling”. The signal thus digitized, filtered and rescaled is then used for demodulating the transmitted communication channel(s) 108, 109, 110, 111, 112. As an example, the GNSS (Global Navigation Satellite System) radio navigation systems usually use broadband signals generated using the spread-spectrum technique. In this case, a communication channel is associated with a spread code.
In order to control the amplitude of the analog signal at the input of the CAN converter, the AGC loop 102 comprises a module 103 for determining the power at the output of said converter and/or the mean of this power and compares it 104 with a control setpoint g1. The difference between the measured power and the setpoint is used by a corrector 105 in order to determine an analog gain ga 106 to be applied to the variable gain amplifier 100 of the analog stage 113. The corrector consists in applying a gain to said difference before supplying an infinite integrator, never reset to zero. The choice of the control setpoint g1 depends on the optimal efficiency value R for the conversion. The choice of the gain determines the time constant of the AGC loop.
If interference affects the input signal e, the latter must be taken into account by the digital stage 114, but also at the time of the analog-digital conversion 101.
As explained above, the role of the AGC loop is to control the amplification gain introduced by the analog stage so as to make the best use of all of the encoding amplitude of the CAN converter while preventing saturation. Thus the ratio of the useful signal power contained in the received signal over the power of the quantization noise is maximized.
The programming of the AGC loop may be adapted so as to take account of the type of interference that affects the signal. Thus, the AGC loop may be programmed to use a low encoding or a high encoding. There are three types of interference affecting a signal. The first type corresponds to the continuous narrowband interference, the second type corresponds to the pulsed narrowband interference and the third type corresponds to the broadband interference.
Continuous narrowband interference is interference that continuously affects a portion of the band occupied by the useful signal. In this case, a band-stop digital filter makes it possible to eliminate the corrupted frequencies without irremediably damaging the useful signal occupying a broader spectrum. Since the useful signal is spread over a broad band, there remains enough useful signal to be demodulated. For the digital filtering to be effective in the presence of interference, it is necessary to digitize the received signal with a large number of bits, for example with N>6. Specifically, the CAN converter introduces a quantization noise which has to be minimized because this noise is uniformly distributed throughout the sampled band. It can therefore not be eliminated by digital filtering. For the ratio of the useful signal power over the quantization noise power to be maximum, the power of the signal at the input of the CAN converter must occupy the whole range of said converter yet without saturating it. For this, the AGC loop is programmed using a high encoding. FIG. 2 illustrates the ways in which the analog input signal can be adapted to the range of the CAN converter.
The power of the input signal must not exceed a saturation limit 205 specific to the converter. A high encoding signifies that a high control setpoint g1 204 is chosen so that the power of the signal at the input of the converter is as close as possible to the saturation limit 205 while ensuring a saturation margin 202. So long as the total power of the received signal is continuous, the controlling of the AGC loop in high encoding ensures an optimal conversion without saturation and with a maximum signal power ratio over quantization noise.
The pulsed narrowband interference is intermittent interference that affects only a portion of the band occupied by the useful signal. Faced with this type of interference, the AGC loop does not have the time to react to control the range of the signal at the input of the analog-digital converter. There is saturation and the consequence of this saturation is that the interfering power is spread throughout the band. In this case, the digital filtering becomes inoperative and a corrupted signal is found at the output. This is why it is preferable in this case to use a low encoding for the AGC loop. In other words, a low setpoint 203 of AGC is chosen in order to retain a maximum saturation margin for the pulsed interference. The power of the input signal must be sufficiently great relative to the quantization noise 206. For this, the input signal is adapted by taking account of a quantization margin 201 so as not to substantially degrade the signal-to-noise ratio after digital conversion. A low encoding signifies that a low setpoint g1 203 is chosen so that the power of the signal at the input of the converter is as low as possible while ensuring a sufficient quantization margin 201. The drawback of low encoding is that, in the presence of narrowband continuous interference, the quantization noise level rises relative to the useful signal level.
In the presence of narrowband interference, the reception performance therefore depends on the choice between high encoding 204 and low encoding 203, that is to say on the choice between a high setpoint for the best processing of the continuous narrowband interference and a low setpoint for the pulsed narrowband interference.
The broadband interference is interference that occupies the whole useful band of the signal. A band-stop digital filter therefore does not make it possible to sort between the frequencies corrupted by this interference and those that are not. In this case, the choice of a high encoding or of a low encoding is of no importance.
Moreover, when there is too much pulsed interference, the mean power at the output of the CAN converter increases. This has the effect of reducing the amplification gain ga by the retroaction 106 of the AGC loop and therefore of raising the level of quantization noise relative to the useful signal.
FIG. 3 illustrates an existing technique making it possible to prevent this phenomenon. The transmission chain of FIG. 3 comprises an analog stage consisting of a variable gain amplifier 300 having the objective of amplifying an analog input signal e. After amplification, the signal is inserted into a CAN converter 301 on N bits. The digitized signal is filtered with the aid of an interference-suppressing digital filter 307. The bits at the output of the filter are rescaled 308 on M bits. The signal is then used to demodulate the received communication channel(s) 308, 309, 310, 311, 312. In order to control the range of the analog signal at the input of the CAN converter 301, a AGC loop 302 determines 303 a signal power, compares 304 this power with a control setpoint g1 and determines a difference between this power and the setpoint g1. A corrector 305 is then used to determine the analog gain ga to be applied 306 to the variable gain amplifier 300. In contrast with the reception chain shown in FIG. 1, the AGC loop 302 is based on the signal at the output of the interference-suppressing filter 307 cleared of the narrowband interference in order to estimate 303 the power.
The drawback of this solution is that there is no control of the power at the output of the CAN converter 301. Therefore, in the event of powerful continuous narrowband interference, the CAN converter 301 begins to saturate before the AGC loop 302 reacts. After saturation, the interference spread throughout the band reappears at the output of the interference-suppressing filter, the loop reacts too late. The use of the output signal of the interference-suppressing filter to estimate the power used in the AGC loop 302 makes it possible to indirectly control the power of the signal excluding interference at the input of said filter because the relationship between the input power and the output power is known in the case of a white noise. Nevertheless, this does not make it possible to control what happens at the input of the CAN converter 301, in particular the risks of saturation.